serial: amba-pl011: Introduce skip_ibrd_fbrd vendor flag

The NVIDIA Tegra264 UART has a broken fractional baud rate
divisor register. Using IBRD and FBRD may cause the baud rate
to fall outside the required tolerance.

Introduce the skip_ibrd_fbrd vendor flag to skip IBRD/FBRD
programming. When set, the baud rate is derived directly from the
UART clock rate using a fixed divisor.

Signed-off-by: Kartik Rajput <kkartik@nvidia.com>
Link: https://patch.msgid.link/20260225065915.341522-2-kkartik@nvidia.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index 462a8c380059385a5499c1fb001f3324b84f1aa7..b6d881eb87ccfe462129d6d4355b31fd932ff643 100644 (file)
--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c
@@ -114,6 +114,7 @@ struct vendor_data {
        bool                    cts_event_workaround;
        bool                    always_enabled;
        bool                    fixed_options;
+       bool                    skip_ibrd_fbrd;
 
        unsigned int (*get_fifosize)(struct amba_device *dev);
 };
@@ -2115,11 +2116,6 @@ pl011_set_termios(struct uart_port *port, struct ktermios *termios,
                uap->dmarx.poll_rate = DIV_ROUND_UP(10000000, baud);
 #endif
 
-       if (baud > port->uartclk / 16)
-               quot = DIV_ROUND_CLOSEST(port->uartclk * 8, baud);
-       else
-               quot = DIV_ROUND_CLOSEST(port->uartclk * 4, baud);
-
        switch (termios->c_cflag & CSIZE) {
        case CS5:
                lcr_h = UART01x_LCRH_WLEN_5;
@@ -2190,21 +2186,28 @@ pl011_set_termios(struct uart_port *port, struct ktermios *termios,
                        old_cr &= ~ST_UART011_CR_OVSFACT;
        }
 
-       /*
-        * Workaround for the ST Micro oversampling variants to
-        * increase the bitrate slightly, by lowering the divisor,
-        * to avoid delayed sampling of start bit at high speeds,
-        * else we see data corruption.
-        */
-       if (uap->vendor->oversampling) {
-               if (baud >= 3000000 && baud < 3250000 && quot > 1)
-                       quot -= 1;
-               else if (baud > 3250000 && quot > 2)
-                       quot -= 2;
+       if (!uap->vendor->skip_ibrd_fbrd) {
+               if (baud > port->uartclk / 16)
+                       quot = DIV_ROUND_CLOSEST(port->uartclk * 8, baud);
+               else
+                       quot = DIV_ROUND_CLOSEST(port->uartclk * 4, baud);
+
+               /*
+                * Workaround for the ST Micro oversampling variants to
+                * increase the bitrate slightly, by lowering the divisor,
+                * to avoid delayed sampling of start bit at high speeds,
+                * else we see data corruption.
+                */
+               if (uap->vendor->oversampling) {
+                       if (baud >= 3000000 && baud < 3250000 && quot > 1)
+                               quot -= 1;
+                       else if (baud > 3250000 && quot > 2)
+                               quot -= 2;
+               }
+               /* Set baud rate */
+               pl011_write(quot & 0x3f, uap, REG_FBRD);
+               pl011_write(quot >> 6, uap, REG_IBRD);
        }
-       /* Set baud rate */
-       pl011_write(quot & 0x3f, uap, REG_FBRD);
-       pl011_write(quot >> 6, uap, REG_IBRD);
 
        /*
         * ----------v----------v----------v----------v-----
@@ -2374,6 +2377,7 @@ static void pl011_console_get_options(struct uart_amba_port *uap, int *baud,
                                      int *parity, int *bits)
 {
        unsigned int lcr_h, ibrd, fbrd;
+       unsigned int clkdiv;
 
        if (!(pl011_read(uap, REG_CR) & UART01x_CR_UARTEN))
                return;
@@ -2393,10 +2397,15 @@ static void pl011_console_get_options(struct uart_amba_port *uap, int *baud,
        else
                *bits = 8;
 
-       ibrd = pl011_read(uap, REG_IBRD);
-       fbrd = pl011_read(uap, REG_FBRD);
+       if (uap->vendor->skip_ibrd_fbrd) {
+               clkdiv = 64;
+       } else {
+               ibrd = pl011_read(uap, REG_IBRD);
+               fbrd = pl011_read(uap, REG_FBRD);
+               clkdiv = 64 * ibrd + fbrd;
+       }
 
-       *baud = uap->port.uartclk * 4 / (64 * ibrd + fbrd);
+       *baud = uap->port.uartclk * 4 / clkdiv;
 
        if (uap->vendor->oversampling &&
            (pl011_read(uap, REG_CR) & ST_UART011_CR_OVSFACT))